Oligohydramnios refers to reduced amniotic fluid volume, often less than 500 mL at 32-36 weeks of gestation. It is also characterized by maximum vertical pocket of less than 2 cm. Amniotic fluid index (AFI) measures less than 5 cm.
Oligohydramnios is characterized by AFI less than 5cm, or the absence of a fluid pocket of 2-3 cm. Deformation of the fetus is characteristic of oligohydramnios caused by fetal renal abnormalities including renal agenesis or dysgenesis. As the fetus is confined to a small space with restricted movement, facial deformation, epicanthal folds, hypertelorism, low-set ears, crease below the lower lip, and micrognathia are also seen. Thoracic compression may also be present as one of the features. Fetal lung development is adversely affected leading to lung hyperplasia. Fetal deformations including bowed legs, clubbed feet, single umbilical artery, gastrointestinal atresias, and a narrow chest are also manifestations. Children born are generally small in stature . Newborn may also present with multicytic-dysplastic kidney, enlarged urinary bladder, or prune-belly syndrome.
Oligohydramnios is often found incidentally during ultrasonography, and this is the best confirmatory test for this condition. A significant lag can be noted in the fundal height when compared to size expected for the gestational period  . Renal agenesis can be observed from fluid-filled bladder and normal kidneys. This will also help in ruling out chances of cystic dysplasia, and uretral obstruction. Subjective criteria for diagnosis of oligohydramnios include absence of fluid pockets in uterine cavity, crowding of fetal limbs in the cavity, absence of pockets around the legs of fetus, and in more severe cases, overlapping of ribs in fetus. Among the objective methods of measurement, single deepest pocket and AFI are the most commonly used .
Oligohydramnios is characterized by AFI less than 5 cm, but occasionally 8 cm or less is considered as the susceptible range. Single deepest pocket in this condition is less than 2 cm. As this value reduces below 2 cm, risk of perinatal morbidity increases considerably. Studies report that both AFI and single deepest pocket (SDP) measurements provide equal diagnostic clarity for oligohydramnios. If the patient is pregnant at an early gestational age, SDP is found to be more accurate. This is true of twin gestations also. In the second trimester, measurement of the deepest vertical pocket is found to be more accurate. In the third trimester AFI is considered to be more sensitive .
Range of motion of the fetus is assessed using sterile speculum examination. Microscopic observation may reveal the presence of arborization by posterior vault fluid. Amniotic fluid will have a basic pH of around 6.5-7.0. MRI and three dimensional ultrasonography are also suggested in the evaluation of amniotic fluid volume . Amniotic wrinkle refers to a possible error in the ultrasonography of twin pregnancies, where one twin actually may have very little amniotic fluid leading to oligohydramnios. This can be avoided by showing intertwin membrane in all images .
Identifying the etiological factor is very important in deciding the treatment strategy for this condition. Maternal bedrest and adequate hydration help in improving the intravascular space. This aids in increasing the production of amniotic fluid, controlling the symptoms. Oral hydration is found to increase AFI by almost 30%. Management of the condition may also depend on the gestational age. If the diagnosis is before term, expectant management is more effective, and is mostly based on the condition of the mother and the fetus. Fetal growth and amniotic fluid volume should be continuously monitored. If oligohydramnios is confirmed, fetal heart rate should be continuously assessed during labour.
If the condition is presented at term, delivery may be opted. Based on gestational age, inducibility of cervix of the mother, and the severity of the condition, delivery may be delayed, provided the fetal condition is normal. Amnioinfusion may be used to increase the amount of amniotic fluid at the time of delivery. In this procedure, saline is transcervically infused using intrauterine catheter. Amnioinfusion is not recommended for patients with preterm rupture of membranes. If the cause of the condition is obstructive uropathy, vesico-amniotic shunts may be helpful in diverting fetal urine to amniotic fluid. This method is found to be effective in relieving oligohydramnios. But, this may not improve pulmonary function or renal function of the fetus.
Prognosis of oligohydramnios depends on the etiology, severity of the condition, gestational age, and duration. Patients presenting with oligohydramnios in the second trimester have higher probability of developing structural malformations. Survival rate is also lower in this case when compared to those presenting in the third trimester. Mortality is mainly due to congenital malformations and pulmonary hyperplasia. Those presenting in the third trimester may not have a bad perinatal outcome.
Etiology of oligohydramnios can be categorized into fetal causes, placental causes and maternal etiological factors. Most common fetal causes of the condition include chromosomal factors, congenital factors, intrauterine growth restriction, post-term pregnancy, premature ROM and death of fetus. Fetal renal abnormalities including agenesis, dysplasia, or obstructive disorders, lead to oligohydramnios because of decreased urine output . Placental factors of oligohydramnios include abruption, placental insufficiency, and twin-to-twin transfusion syndrome. Maternal causes include dehydration, uteroplacental insufficiency, hypertension, preeclampsia, diabetes mellitus, and chronic hypoxia. Post-maturity syndrome that results with the extension of pregnancy beyond 42 weeks, may also result in oligohydramnios. Use of certain medications, particularly prostaglandin synthase inhibitors or ACE inhibitors are also implicated in the development of oligohydramnios.
Incidence of oligohydramnios is about 4.5% of all pregnancies. The chance of severe form of this condition is 0.7% in all pregnancies . Incidence of oligohydramnios is more common in pregnancies that continue beyond term, as the amniotic fluid volume diminishes with term. Oligohydramnios is a complication in about 12% of the pregnancies that continues beyond term. It may occur at any time during gestation and is less common when compared to polyhydramnios .
Oligohydramnios constraints the movement of fetus within the uterus and can result in a cascade of events. The joints may be completely or partially immobilized in the confined space and which may result in congenital contractures. Space for the development of thorax is reduced and this affects distention of lung tissue leading to lung hyperplasia. Reduced fetal activity in the chamber results in a shortened umbilical cord. Compressive force within the uterus may lead to dysmorphic facial features like micrognathia, low-set ears, small alae nasi, and hypertelorism. Muscle development and weight gain are also affected by the restricted fetal activity in the confined space. As the volume of amniotic fluid diminishes, it may lead to microgastria. Restricted movement of the fetus may also lead to loose skin .
Keeping well hydrated during pregnancy helps to improve the amniotic fluid content and thus deter oligohydramnios.
Oligohydramnios refers to reduced amniotic fluid volume, often less than 500 mL at 32-36 weeks of gestation. It is also characterized by maximum vertical pocket of less than 2 cm. Amniotic fluid index (AFI) measures less than 5 cm. As the volume of amniotic fluid changes with gestational age, AFI is considered to be the best measure for detecting this condition. In oligohydramnios AFI is less than the fifth percentile . In many cases oligohydramnios does not have a clearly defined etiological factor. It may result from a premature rupture of membranes (ROM). Increased fluid intake by the mother is found to increase amniotic fluid volume. It may lead to umbilical cord depression and fetal distress. In most of the cases a decision may be required to see whether the fetus should remain within the uterus or not.
Oligohydramnios is a condition characterized by reduced levels of amniotic fluid in the uterine cavity. The amount of fluid is measured using amniotic fluid index (AFI) or deep pocket measurements in the cavity. About 4.5% of pregnant women may develop this condition. Around 8% of pregnant women have an increased risk of developing this condition. Although oligohydramnios may develop during anytime of pregnancy, it is more common during the last trimester. Those who are beyond the term also have an increased risk of this condition, as amniotic fluid reduces by half after 42 weeks of gestation. Oligohydramnios may lead to many complications.
Oligohydramnios may be caused by birth defects like kidney problems in the fetus that lead to reduced production of urine, which in turn affects the production of amniotic fluid. Problems in placenta may affect the recycling of fluid, reducing the amount. Rupture of membranes leads to a loss of fluid. Functioning of placenta reduces post the date of pregnancy and hence gestation beyond 42 weeks may have low amniotic fluid volume. Maternal factors like dehydration, hypertension, diabetes, and chronic hypoxia ma also lead to the development of this condition. As amniotic fluid is essential for the growth and development of fetus, it may affect the formation of muscles, limbs, lungs and digestive system. Complications are severe if oligohydramnios is detected in the first trimester. It may lead to compression of fetal organs, and increases the chance of still birth or miscarriage. When detected in the second trimester, it may lead to complications like pre-term birth, intrauterine growth restriction, and complications during delivery.
Treatment strategies are based on the actual cause of the condition and also the gestational age. If term is not complete, continuous monitoring of fetal health would be required. If it is close to full term, delivery is usually the option. Amnioinfusion is a method in which fluid is infused into the uterine cavity using a catheter. This helps to reduce the chances of cesarean delivery. Maternal rehydration with oral or IV fluids is effective in increasing the amniotic fluid volume, decreasing complications. Fluids may also be injected prior to delivery through amniocentesis.
Outcome of oligohydramnios is poor if it is detected in the first and second trimester. Fetal mortality rates are high when this condition is detected in the second trimester. This is mostly due to congenital malformations and abnormal lung functioning.